The present invention pertains generally to the extraction of a constituent from a chemical mixture. More particularly, the present invention pertains to plasma devices and methods for ionizing the constituents of a chemical mixture and separating the resulting ions according to their relative mass to charge ratio. The present invention is particularly, but not exclusively, useful for generating an ion beam containing one or more selected ions from a chemical mixture such as a metallic alloy.
For applications wherein the purpose is to separate a constituent element from a chemical mixture such as a metallic alloy or some other mixture of elements, there are several possible ways to proceed. In some instances, mechanical separation may be possible. In others, chemical separation may be more appropriate. For cases where the chemical mixture includes alloying elements from the same chemical group (i.e. alloying elements from the same column in the periodic table), chemical separation is often difficult or impossible due to the similar chemical properties of the alloying elements. An example of a difficult to separate mixture is a metallic alloy of Zirconium (Zr) and Hafnium (Hf).
When mechanical or chemical processes are not feasible, it may happen that separation procedures and processes involving plasma physics may be necessary. Specifically, a plasma can be made from the chemical mixture and the resultant ions separated according to their respective mass to charge ratio. Ion separation can be accomplished in several ways known in the pertinent art. For example, plasma centrifuges and their methods of operation are well known. On the other hand, and not yet so well known, plasma filters and their methods of operation are also useful for this purpose. For example, the invention as disclosed by Ohkawa in U.S. Pat. No. 6,096,220 which issued on Aug. 1, 2000, for an invention entitled xe2x80x9cPlasma Mass Filterxe2x80x9d and assigned to the same assignee as the present invention is useful for separating ions of different mass to charge ratios.
Another example of a plasma separation process was disclosed and claimed in U.S. patent application Ser. No. 09/630,847 entitled xe2x80x9cMass Filtering Sputtered Ion Sourcexe2x80x9d by S. Putvinski and V. Volosov, which was filed on Aug. 2, 2000 and is assigned to the same assignee as the present invention. In the ""847 application, a separation process is disclosed wherein a central cathode is prepared from the mixture requiring separation. Ions from an anode that surrounds the central cathode are used to bombard the central cathode, sputtering the mixture into a plasma chamber where the mixture dissociates and ionizes. Crossed electric and magnetic fields that are established in the plasma chamber cause ions having a relatively high mass to charge ratio to fall back onto the central electrode, sustaining the sputtering of the central electrode. On the other hand, ions of relatively low mass to charge ratio respond differently to the crossed electric and magnetic fields. Specifically, these ions do not fall back onto the central electrode, but rather, they are confined inside the chamber for subsequent removal from the plasma through an exit formed in the chamber. It happens, however, that not all chemical mixtures are efficiently separated by sputtering the mixture from a central electrode.
In light of the above, it is an object of the present invention to provide devices and methods suitable for the purposes of extracting a constituent from a chemical mixture such as a metallic alloy. It is another object of the present invention to provide novel devices and methods for the separation of chemical mixtures that were previously inseparable or difficult to separate using existing separation techniques. Still another object of the present invention is to provide energy efficient methods and devices for separating chemical mixtures into their constituent elements. It is yet another object of the present invention to provide methods and devices for separating a metallic alloy into its constituent elements and creating a substantially pure ion beam containing only one of the alloying elements. Yet another object of the present invention is to provide devices and methods for separating a chemical mixture into its constituents which are easy to use, relatively simple to implement, and comparatively cost effective.
The present invention is directed to devices and methods for separating a chemical mixture into its constituents. In overview, the present invention includes a central cathode that is aligned axially within a cylindrical plasma chamber. An anode, made of the chemical mixture requiring separation is positioned near the cylindrical wall of the plasma chamber. A working gas, which is preferably a noble gas, is introduced into the chamber near the cylindrical wall to sputter the chemical mixture (i.e. the anode) into the plasma chamber where it is dissociated and ionized. To reduce the unwanted loss of the central cathode due to sputtering by the working gas, the central cathode is formed with a plurality of radial projections that extend outwardly from the axis of the cylindrical plasma chamber. The functional effect of these radial projections is to capture sputtered cathode material before it is lost to the plasma. On the other hand, once the chemical mixture has been sputtered from the anode and ionized in the plasma chamber, its ions are separated, according to their respective mass to charge ratio, using crossed electric and magnetic fields.
In greater detail, the device of the present invention includes a cylindrical plasma chamber that is open at one end, closed at the other end and defines a longitudinal axis. The plasma chamber has a cylindrical wall that is formed with a plurality of holes to allow the working gas to be introduced into the chamber. Inside the plasma chamber, an elongated central cathode is positioned and oriented substantially along the longitudinal axis. Preferably, the central cathode is hollow to accommodate recycling of the working gas that accumulates at the central cathode. For this purpose, one or more channels are formed in the central cathode to allow working gas in the plasma chamber to enter the hollow central cathode. From the hollow central cathode, the working gas is routed outside the plasma chamber for subsequent reintroduction into the plasma chamber through the holes in the cylindrical wall.
In accordance with the present invention, crossed electric and magnetic fields are established in the plasma chamber. These fields are used to cause the working gas to sputter the chemical mixture (i.e. the anode), and to separate the resulting ions according to their respective mass to charge ratio. To generate the required magnetic field in the chamber, coils are mounted on the outside of the cylindrical wall. With this cooperation of structure, a magnetic field that is oriented parallel to the longitudinal axis is created. The coils are preferably configured to create a magnetic field having a substantially uniform field strength throughout the chamber.
In accordance with the present invention, a radially oriented electric field is established in the plasma chamber. To create the electric field, ring electrodes are positioned in the plasma chamber near the closed end of the cylindrical wall. In some embodiments, a voltage source can be connected directly to the wall and the central cathode to establish part or all of the required electric field. Importantly, the radially oriented electric field is directed inwardly from the wall towards the central cathode.
To separate the chemical mixture into its constituents, the chemical mixture is positioned in the plasma chamber near the cylindrical wall. To accomplish this, the chemical mixture can be formed as one or more tiles that are mounted on the inside surface of the wall, facing the central cathode. Alternatively, the wall itself can be made of the chemical mixture. In either case, the chemical mixture is electrically connected to the voltage source to effectively become an anode relative to the central cathode.
In the operation of the present invention, the chemical mixture requiring separation is first identified. Generally, the chemical mixture will contain two constituents for separation, with one constituent having a relatively low mass to charge ratio in the plasma and the other constituent having a relatively high mass to charge ratio in the plasma. Next, the working gas is selected. For the present invention, the working gas is preferably selected to have a mass to charge ratio in the plasma that is between the mass to charge ratios of the two constituents of the chemical mixture.
Once selected, the working gas is introduced into the chamber through the holes that are provided in the cylindrical wall. With the plasma chamber filled with working gas, the ring electrodes are energized to establish a radial electric field in the chamber that is sufficient to create a plasma from the working gas. Once a plasma has been initiated in the chamber, the strengths of the electric and magnetic fields can be adjusted to effect the sputtering of the anode by the working gas and the separation of the resulting ions according to their respective mass to charge ratio. Specifically, under the influence of the electric and magnetic fields, molecules of the working gas are ionized near the cylindrical wall and directed on trajectories towards the central cathode. Near the central cathode, the ions of the working gas undergo electron exchange reactions with neutral atoms that are present there, creating fast neutrals that are directed towards the anode. Importantly, the fast neutrals are produced with sufficient energy to strike the anode and thereby sputter the chemical mixture into the plasma.
In the plasma, the sputtered chemical mixture is dissociated into its constituents, and the constituents are ionized. Due to the strengths and orientations of the electric and magnetic fields in the chamber, ionized constituents having a relatively high mass to charge ratio are placed on trajectories that are directed towards the central cathode (i.e. orbital trajectories of large radius). Upon striking the central cathode, these ions are captured. On the other hand, ions having a relatively low mass to charge ratio are placed on small radius, orbital trajectories. As such, these ions are directed out of the plasma chamber through the open end. The result is an essentially pure ion beam exiting from the device that contains only the ions of relatively low mass to charge ratio.